The advent of elastomeric pump devices has made infusion possible in non-hospital based, ambulatory settings. Typically the construction of such device consists of an elastomeric bladder that stores the energy when filled with drug of fluid. This energy that is derived from the expanded bladder pushes the fluid out via tubing when the bladder compresses. The flow rate is controlled by appropriate sized lumen of a PVC tube or orifice of a glass capillary. Such single-use devices have the advantages of convenience, safety and without the complications of electronics.
One major drawback of elastomeric pumps is the difficulty to achieve accurate flow accuracy throughout the entire flow duration. Referring to FIG. 1, a typical flow profile (i.e. flow rate vs. time) of an elastomeric pump shows that the flow rate of fluid from such a pump is not constant during the flow duration because the pressure exerted on the fluid in the pump is not constant due to the inherent characteristics of such pumps. Flow begins with an inherent initial spike 20, continues with a trough shaped phase 21 having a lower flow rate, and finishes with a slight second spike 22. While the troughs and peaks of such a profile may be averaged to provide a sufficient flow rate, in some cases, such as for the delivery of toxic medication or when a narrow therapeutic range is required, the initial spike may result in an overdose or another undesirable situation. To overcome this, some manufacturers recommend that such pumps be left aside for an initial period, presumably for the elastomer to relax before infusion starts.
Another drawback of elastomeric pumps is that the energy stored in the bladder is dependent on the fluid fill volume. Depending on the geometry and intrinsic characteristics of the material used, variations in fill volume may result in changes in flow rate to a varying extent. This dependency of flow rate and flow duration on fill volume is evident in the relatively fixed fill volume that is specified for any such pump. Only small variations in fill volume are permitted if the specified flow rate is to be attained.
The present invention addresses the above issues by making possible infusion delivery of fluid with a constant flow accuracy that is unaffected by the non-uniform pressure exerted by the expanded bladder in elastomeric devices. This non-uniform pressure is due to the changing fill state of the conventional bladder as its content is delivered and also to some extent on the duration the content remains in the bladder which could affect its elasticity. As pressure generated in the present invention is independent of the filled state and that there is no impact of the duration during which the content remains in the device, the performance characteristics of the device can be optimized relatively easily at its design stage.